The invention relates to an apparatus for measuring the level of the interface between two different media in a reservoir comprising a vibratory element adapted to contact both media on all sides, and an amplifier circuit which together with the vibratory element forms an oscillator circuit in which the vibratory element is the frequency determining part, wherein the vibratory element makes flexural vibrations at a resonance frequency, and a processing unit for measuring the momentary resonance frequency of the vibratory element and for deriving the level of the interface with respect to the vibratory element from said momentary resonance frequency by means of the predetermined dependency of the resonance frequency on the depth of immersion of the vibratory element in the lower medium.
Such an apparatus is disclosed in the article "Vibratory proces control transducers" by R. M. Langdon, Electronic Engineering, vol. 53, nr. 659, November 1981, pages 159-168. In this known apparatus the vibratory element is made as a rigid metal rod or tube with a piezo-electric transducer attached to it to generate flexural vibrations. This rod or tube extends along the whole height of the reservoir and the resonance frequency is approximately linearly dependent on the fraction of the rod or tube immersed in one of the mediums. However in view of the required length of the rod or tube, the rod or tube is vibrating at a relatively high frequency so that a number of standing waves will be present on the tube which produce vibrational nodes at intervals along its length. Such vibrational nodes have the disadvantage that the sensitivity of frequency to level change is low in the vicinity of these nodes. Compensating this sensitivity variation is rather complicated. Moreover, the accuracy of the level measurement is relatively low.